LEYS, S.P.: The hexactinellid body plan: syncytial metazoans Hexactinellids differ from other sponges in possessing mostly syncytial rather than cellular tissues. The major tissue component, termed the trabecular reticulum, forms a cobweb-like continuous network throughout the entire sponge. The trabecular reticulum is bilayered and encloses a thin collagenous mesohyl in which lie several ‘cellular’ components […]

M�LLER, W.E.G.: The Urmetazoa: The hypothetical ancestral animal The origin of Metazoa remained the most enigmatic of all phylogenetic problems. Sponges [Porifera] as “living fossils” positioned at the base of multicellular animals have been used to answer basic questions in metazoan evolution by molecular biological techniques. During the last few years genes coding for informative […]

VALENTINE, J.W.: The Evolutionn of Genomic and Morphological Complexity in Metazoans: The Topology of Gene Activities A useful measure of the complexity of an object or process is the size of its minimum description. For metazoan bodies, a thorough description would have to begin with subatomic particles and encompass all the constructional levels of the […]

WAGGONER, B.M.: The Ediacara biota in space and time The enigmatic “Ediacara organisms”, which preceded and overlapped with the Cambrian radiation of metazoans, include many forms whose systematic positions remain contentious after over fifty years of study. It might seem that nothing particularly useful can be learned from a biota full of oddballs. However, preliminary […]

CONWAY MORRIS, S.: The Cambrian explosion: what’s the problem? Behold the donkey approaching; hardly able to move under the impedimenta. Leading it is the bowed figure of Darwin, and on its back a number of luminaries waving manuscripts; while trudging behind, with a large scoop? – well, never mind. Now let us creep closer. What […]

GERSHWIN, L.*; LIPPS, J. H.: Some new thoughts on some old animals The Ediacaran fauna has been subject to diverse and sometimes wild interpretations, especially during the last decade or so when it has become fashionable to interpret the group as non-metazoan. Our investigations have shown that some forms can be better understood in terms […]

FEDONKIN, M.A.: Megascopic trace and body fossils of the oldest metazoans Bilateral body fossils Dickinsonia, Ovatoscutum, Chondroplon, Yorgia and Andiva from the Vendian 555 Ma old deposits (White Sea) have the shield-like form, bipolarity, complete and homonomous metamerism combined with the symmetry of gliding reflection. Modes of deformation, growth pattern and the regeneration marks indicate […]

JACOBS, David K; GATES, Ruth D: Is reproductive signaling antecedent to metazoan sensory and neural organization? We explored the POU/Homeodomain containing developmental genes down the tree of life, recovering bilaterian, cnidarians, ctenophore and sponges sequence. The genes in question include a sense organ specific gene – Brain3, a pituitary specific gene – Pit1 and a […]

CARTWRIGHT, Paulyn: Hox genes, Hydroids, and Colonial Integration A trend toward increased colony-wide integration is a common theme in the evolution of colonial metazoans. This trend is nowhere more apparent than within the Cnidarian class, Hydrozoa, where colonial forms range from a simple array of individual polyps, to a highly integrated, complex organization of polyps. […]

HAGEMAN, S.J.: Growth patterns and complexity in colonial (modular) organisms Complexity in colonial organisms can be described in terms of the degree of integration of polymorphs (a collection of identical, independent individuals sharing a colony structure, vs. a highly integrated collection of specialized individuals whose interdependence results in a “super-organism”). A second level of complexity, […]